A. Summary of Positive Pressure CPAP Machines
Since the invention of nasal Continuous Positive Airway Pressure (nasal CPAP) for treatment of Obstructive Sleep Apnea (OSA) and other forms of Sleep Disordered Breathing (SDB) by Sullivan, as taught in U.S. Pat. No. 4,944,310, much effort has been directed towards improving the comfort of patients using the devices by controlling the pressure supplied to masks such as the MIRAGE® and ULTRA MIRAGE® manufactured by ResMed Limited.
Some CPAP blower devices, such as the S7™ and S8™ device by ResMed Limited, provide a supply of air at a fixed positive pressure throughout the respiratory cycle of a patient, for example, 15 cm H2O. A blower comprising an electric motor and fan can be constructed to deliver a particular pressure to a patient interface, such as a mask. When the patient breathes in with such a system, the pressure in the mask may reduce by a small amount. When the patient breathes out with such a system, the pressure in the mask may increase by a small amount. These fluctuations in mask pressure are referred to as “swing”. Some blowers use feedback in a pressure controller which counterbalances the effect of patient effort on the mask pressure to reduce the swing. These devices require measuring and monitoring the mask pressure and flow and adjusting the flow generator control to maintain the mask pressure at a constant value.
B. Need for Accurate Mask Pressure Determination
What is required for each of these devices is a method to determine the pressure and flow at the patient interface. In order to accurately determine pressure and flow at a mask, one either measures them at the mask or measures them near the pressure generator and corrects for various factors, one of which is the pressure loss in a length of tubing between a pressure sensor and the mask.
i. Pressure Correction Due to Losses in the Tubing
U.S. patents that have discussed the pressure loss in tubing of CPAP devices are U.S. Pat. No. 6,817,361 entitled “Administration Of CPAP Treatment Pressure In Presence Of Apnea”, U.S. Pat. No. 6,810,876 entitled “Assisted Ventilation To Match Patient Respiratory Need”, U.S. Pat. No. 6,688,307 entitled “Methods And Apparatus For Determining Instantaneous Elastic Recoil And Assistance Pressure During Ventilatory Support”, U.S. Pat. No. 6,675,797 entitled “Determination Of Patency Of The Airway”, U.S. Pat. No. 6,575,163 entitled “Method For Calculating The Instantaneous Inspired Volume Of A Subject During Ventilatory Assistance”, U.S. Pat. No. 6,532,957 entitled “Assisted Ventilation To Match Patient Respiratory Need”, U.S. Pat. No. 6,502,572 entitled “Administration Of CPAP Treatment Pressure In Presence Of APNEA”, U.S. Pat. No. 6,484,719 entitled “Method For Providing Ventilatory Assistance In A Spontaneously Breathing Subject”, U.S. Pat. No. 6,367,474 entitled “Administration Of CPAP Treatment Pressure In Presence Of APNEA”, U.S. Pat. No. 6,363,933 entitled Apparatus And Method For Controlling The Administration Of CPAP Treatment”, U.S. Pat. No. 6,138,675 entitled “Determination Of The Occurrence Of An Apnea”, U.S. Pat. No. 6,029,665 entitled “Determination Of Patency Of Airway”, U.S. Pat. No. 5,704,345 entitled “Detection Of Apnea And Obstruction Of The Airway In The Respiratory System”, and U.S. Pat. No. 5,551,419 entitled “Control For CPAP Apparatus”.
These patents propose an alternative to measuring air flow and mask pressure at or near the mask by mounting flow and pressure transducers near the air pressure generator and then calculating the pressure loss along the tubing from the air pressure generator to the mask from the flow through the tubing and a knowledge of the pressure-flow characteristic of the tubing, for example, by table lookup. The pressure at the mask is then calculated by subtracting the tube pressure drop from the pressure at the pressure generator.
The pressure loss from a pressure measuring point to the mask has been calculated from the flow at the blower and the (quadratic) resistance from the measuring point to the mask according to the formulaΔP=R*Q^2,where ΔP is the hose pressure drop, R is the hose resistance, and Q is the flow. The mask pressure is then calculated by subtracting the hose pressure drop from the measured sensor pressure. In order to use this technique, a flow sensor is necessary, for example, a pneumotachograph and differential pressure transducer. See, e.g., U.S. Pat. No. 6,810,876 at col. 17, lines 25-50.
U.S. Pat. No. 5,551,419 also recognizes that the air pressure in the mask is a function of the pressure inside the base unit housing, the pressure generator and the pressure drop in the delivery hose. It describes the latter as a function of the flow through the hose and concludes that it is necessary to combine the pressure signal and the flow signal to produce a signal that accurately represents the pressure at the mask. See U.S. Pat. No. 5,551,419 at column 5, lines 7-12.
In order to maintain a steady mask pressure the pressure drop along the tube is added to the desired set pressure at the mask to yield the desired instantaneous pressure at the pressure generator. In some cases the controller of the pressure generator has a negative feedback input from the pressure transducer so that the desired pressure is achieved more accurately. See, e.g., U.S. Pat. No. 5,704,345 at col. 8, lines 26-55.
ii. Failure to Manage Swing
One important factor for patient comfort that must be managed is the swing. For example, for the ResMed S8 inhalation and respiratory therapy devices to ensure particular pressure stability requirements are met, the pressure difference between the inspiration and expiration phases, i.e. the swing, must not exceed 0.5 hPa (“hectopascals”) to meet German MDS specifications. 1 hPa=1.04 cm H2O. The tolerance limits for inspiration and expiration fluctuations from set ventilation pressure areVentilation pressure<10 hPa: *p<=0.5 hPaVentilation pressure>=10 hPa: *p<=1 hPa.The prior art techniques for determining mask pressure when the pressure sensor is not at the mask have failed to control swing consistent with these requirements. What is needed is a more accurate determination of mask pressure so that the information may be fed back to the pressure generator in order to achieve better control of swing.
iii. Failure to Predict Dynamics
The mask pressure measurement can also be used in other therapy and control related algorithms. One such algorithm is the discrimination of closed and open respiratory apneas using forced oscillations (e.g. David Bassin's “Discriminating Closed and Open Respiratory Airway Apneas by Forced Oscillation Measurements at the Flow Generator” US Provisional Application 60/823,973). This algorithm requires accurate determination of the mask pressure and flow. The hose drop model presented in the prior art fails to accurately model the dynamic characteristics of the air delivery system and as such fails to accurately model the mask pressure and flow.